Issue Date: March 31, 2014 | Web Date: March 27, 2014
A change is in the works for self-assembled monolayers (SAMs). This 30-year-old technology used for surface protection, sensing, microelectronics, and drug delivery has relied almost exclusively on modifying gold surfaces with a uniform single layer of long-chain alkanethiols.
But now a Canadian research team has come up with a more stable alternative: N-heterocyclic carbenes (NHCs). These cyclic amines, which dramatically increase the lifetime of SAMs, could enable new applications of SAM-modified surfaces.
In alkanethiol-based SAMs, a lone pair of electrons on sulfur binds to the gold surface and the alkane tail is left free. But thiol-based SAMs aren’t perfect. The sulfur groups start detaching from the gold surface after a week or two or slowly degrade with extended exposure to air, light, and water. The monolayers break down more quickly when heated or exposed to common solvents, such as tetrahydrofuran. This instability has been an impediment to their broader commercial use.
Chemists have tried a host of alternative molecules for preparing SAMs but continue to experience similar stability problems or have been unable to achieve good control over monolayer deposition. As Cathleen M. Crudden and J. Hugh Horton of Queen’s University in Kingston, Ontario, and their colleagues have now found, NHCs are proving to be different.
NHCs are already popular ligands that bind and stabilize transition-metal catalysts through a lone pair of electrons on the central carbon atom. Although NHCs are more reactive than thiols, Crudden notes, they have the advantage of interacting more strongly with gold and remaining inert for longer periods in air, water, high- and low-pH conditions, and even boiling tetrahydrofuran (Nat. Chem. 2014, DOI: 10.1038/nchem.1891 ).
One secret to successfully using NHCs could be starting with a thiol-modified surface, says Tobias Weidner of the Max Planck Institute for Polymer Research, in Mainz, Germany, whose team was the first to report NHC monolayers on gold in 2011. Thiol groups can displace molecular contaminants from gold to create uniformly modified surfaces—a self-cleaning process, Weidner explains. NHCs tend to bind to contaminants instead of displacing them, which may have been a limiting factor in the quality of the NHC-based SAMs reported in the past.
But the Crudden team found that the stronger-binding NHCs displace sulfur groups, leading to modified surfaces. Crudden adds that less bulky NHCs also seem to be critical.
“This work represents an important step forward for NHC-based SAMs,” Weidner says. “What is really exciting about the paper by the Crudden lab is that they avoided the contaminants by using thiol-protected gold surfaces as clean substrates—a very elegant solution.”
The Canadian team has further modified the NHCs with electroactive groups once they are on gold. “We are already getting really good results on biosensing and in protecting reactive metals from corrosion in automotive applications,” Crudden says. The researchers are also making NHC-modified nanoparticles, which could be useful for drug delivery.
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